In digital radio applications, such converters, hitherto, have been limited in frequency to the audio range (baseband) and have therefore been preceeded by analogue down-conversion circuitry, itself a source of error in cross- and intermodulation products of the order of -100 db, i.e. at the lowest acceptable limit for the system.
In a typical arrangement, the baseband signal is sampled at an arbitrarily high rf frequency, and, after digital conversion, a large number of digitised samples accummulated to afford improved resolution or unwanted signal rejection. Such techniques have been described in the patent literature, for example: United Kingdom Patent Applications G.B. Nos. 1,532,538; 1,580,178; 2,123,233; and, 2,144,285; U.S. patent application Ser. No. 3,820,111; and, International Patent Application W.O. No. 86/02217.